A Dynamic Reaction Coordinate calculation [29] is to be run. By default, total energy is conserved, so that as the 'reaction' proceeds in time, energy is transferred between kinetic and potential forms. See Dynamic and Intrinsic Reaction Coordinates for more details. Also, by default, all atoms are allowed to move.  When it is necessary to keep some atoms fixed, atoms in the data-set that are not flagged for optimization can be frozen by adding keyword NOOPT.  

By default, steps of 0.1fs are used. This default can be changed using keywords:

To print reaction coordinate steps that are separated by constant time, use T-PRIORITY or T-PRIORITY=n.nn.  This is the option normally chosen.
To print reaction coordinate steps that are separated by constant energy, use H-PRIORITY or H-PRIORITY=n.nn.
To print reaction coordinate steps that are separated by constant distance, use X-PRIORITY or X-PRIORITY=n.nn.

Use LARGE to control print of internal coordinates and Cartesian coordinates.

A useful keyword for limiting the DRC is BIGCYCLES=n.  BIGCYCLES=n will run until n complete oscillations have occurred.   Normal modes of vibration can be generated using BIGCYCLES=1 and IRC=n, where n is the n't normal mode of interest. Such a normal mode would involve one complete oscillation, from equilibrium to turning point back to equilibrium, then up the other side of the potential well to the other turning point, then back to the equilibrium position, at which point the DRC is stopped.

Most of the trajectories mapped out by  BIGCYCLES=1 IRC=n DRC are anharmonic, that is, they reflect the asymmetry of the Morse potential.  They are, however, accurate within the semiempirical approximation used.  Thus if a vibrational frequency is m cm-1, the duration of one complete vibration will be 1/(m.c) seconds.  For example, if a normal mode has an energy (frequency) of 1000 cm-1, each complete vibration would take 33.4 femtoseconds.

Individual DRC paths can be joined together to form a single path using the Windows MOPAC utility program "JOIN"